Adds creationTime to Room attributes

[yepher]

Based on conversation here: https://livekit-users.slack.com/archives/C07FVFGAUKX/p1734195239132809?thread_ts=1733560958.285519&cid=C07FVFGAUKX

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[hiroshihorie]

Hi, thanks for your PR!
I think it might be better to expose it as a Date (UTC) var for better dev experience. 🤔 @bcherry

[bcherry]

agree - that would match the format of Participant.joinedAt